1. Field of the Invention
This invention relates to a magnetic recording medium of a so-called overlapped coating type, and more particularly to improvement in an electromagnetic conversion characteristic and a durability of such a magnetic recording medium.
2. Description of the Prior Art
A magnetic recording media conventionally used, for example, in a video tape recorders (VTRs), include a so-called coating type magnetic recording medium whose magnetic layer is formed by applying a magnetic coating material prepared by mixing a magnetic powder, a binder resin, an organic solvent and other additives together, onto a non-magnetic substrate such as a polyester film, a so-called thin metal film type magnetic recording medium whose magnetic layer is formed by depositing a ferromagnetic material on a non-magnetic substrate using a thin film vacuum-deposition techniques, or the like. At present, the former coating type magnetic recording medium is predominately utilized due to its high productivity and wide applicability.
In such a magnetic recording medium, studies have been made to reduce a thickness of the magnetic layer in order to limit an output loss due to a recording demagnetization to a low level. However, in the case of the magnetic recording medium having only a single layer on a substrate, when a thickness of the magnetic layer is reduced, a non-uniformity on a surface of the substrate remarkably gives an adverse influence on a surface of the magnetic layer, so that the magnetic layer cannot have a sufficiently smooth surface.
Under this circumstance, there has been proposed a overlapped coating type magnetic recording medium having a plurality of coated layers. In such a overlapped coating type magnetic recording medium having a plurality of coated layers, an upper layer is formed as a magnetic layer and lower layers other than the upper layer can be a magnetic or non-magnetic layer. In such a magnetic recording medium, since the non-magnetic substrate is not in direct contact with the upper magnetic layer, a shape of the surface of the upper magnetic layer is not adversely affected by that of the surface of the non-magnetic substrate, so that a thin magnetic layer having a smooth surface can be formed on the magnetic recording medium.
Further, in the overlapped coating type magnetic recording medium, it is important to assure a traveling ability and a durability thereof. For instance, there is a suggestion that, if a glass transition point Tg(u) of the binder resin used in the upper magnetic layer and a glass transition point Tg(l) of the binder resin used in the lower layer satisfies the relationship of Tg(u) .gtoreq.Tg(l), the magnetic recording medium having excellent traveling ability and durability can be obtained.
However, in the event that the glass transition points Tg of the binder resins used are limited to such a particular range, the traveling ability and durability of the magnetic recording medium under high-speed frictional contact conditions, for example, in a still play-back condition, cannot be sufficiently improved though excellent traveling ability and durability are obtained under a low-speed frictional contact condition.